Assessing available N from fall and spring applications

Excessive rainfall and flooding in early to late spring can result in the loss of some fall- and spring-applied nitrogen (N). Both of these N forms are subject to leaching through the soil into tile drains or groundwater. In addition, the nitrate form of N can converted to several gaseous forms and lost to the atmosphere from deep within the soil by a bacterial process called denitrification. Unfortunately, no matter what form of N was added to the soil it will eventually become nitrate. Calendar time since N application and spring temperatures influence the extent to which both fall- and spring-applied N convert to the nitrate form. Many factors affect how much N is lost from soil, therefore it is difficult to accurately estimate the amount of N loss that may have occurred by any point in time. One of the viable options to estimate the amount of remaining soil N is to consider soil sampling and analysis for the nitrate and ammonium forms of N.

Soil sampling strategies

Collect soil cores to a depth of at least 1 foot. Where earlier-applied fertilizer N was broadcast rather than banded, collect 20 to 30 soil cores per sample. Where earlier-applied fertilizer N was banded (e.g., anhydrous ammonia), collect 15 to 20 soil cores using the sampling scheme illustrated in Figure 1. Consider collecting a separate deeper soil sample from between 1- and 2-foot deep for a more complete assessment of plant available N, especially in sandy soils where leaching through the soil profile is the predominant form of N loss.

Sample handling

Dry or refrigerate the soil samples as soon as possible to stop the soil microbes from altering the N levels. Spread the soil thinly on plastic to air dry and hasten drying with a fan if possible. If you choose to use an oven to dry the soil, keep the temperature below 250F. Alternatively refrigerate the samples and keep them cold through shipping to the laboratory. A list of certified soil testing laboratories is available here. Most should offer soil N test analysis services, but contact them first to confirm.

Soil-test laboratory analyses

Ammonium N (NH4-N) is just as available to plants as is nitrate N (NO3-N), but typically little accumulates in the soil because it is readily converted to nitrate under most conditions. However, if N fertilizer was recently applied, there may well yet be some ammonium N available in the soil for plant use.

TIP: When you submit the soil samples to the soil-testing laboratory, request analyses for exchangeable ammonium as well as for nitrate, particularly if anhydrous ammonia was applied relatively recently or a nitrification inhibitor was used with the N fertilizer.

Significant levels of soil ammonium are most likely if anhydrous ammonia was the N source, a nitrification inhibitor such as nitrapyrin or dicyandiamide (DCD) was used, and/or soil pH was low ( below 5.5 ). In these situations, low levels of soil nitrate may indicate little conversion of ammonium to nitrate, rather than simply loss of nitrate.

If soil test values for ammonium and nitrate are reported as ppm or mg/L nitrogen (NH4-N or NO3-N), then pounds per acre of available N are calculated by multiplying the test results by 4 when the sample depth was 1 foot. For other sample depths, divide the sample depth (in inches) by 3 and then multiply by the test results.

In our opinion, soil nitrate and ammonium levels can be used to guide additional N applications to fields subjected to saturation and flooding. However, there are admittedly no hard and fast research-based recommendations for this particular situation.

The primary tool for soil N sampling in the Eastern Corn Belt has been the pre-sidedress soil nitrate test (PSNT) which is most applicable as an indicator of N availability in soils where manure had been applied or a legume such as clover or alfalfa had been plowed down (Brouder & Mengel, 2003). For these field situations, the level of soil nitrate found is considered an index of N availability, i.e., an indicator of how much N is currently available AND how much N may become available from the manure or organic matter. When used in this context, soil NO3-N levels greater than 25 ppm are thought to be adequate for optimum corn yield without the addition of more fertilizer N. During the research that developed this soil test, sampling deeper than 1 foot or analyzing for exchangeable NH4-N did not increase the predictive ability of the PSNT enough to warrant the extra effort.

However, when the intent is to assess the loss of N due to rainfall, we suggest that deeper sampling plus analysis for NH4-N content can provide useful information to help growers decide whether additional fertilizer N is merited. It is important to recognize that in this context, measurements of soil nitrate and ammonium following fertilizer N applications indicate current N availability only, because there is no manure- or legume-derived N to be released later in the season. Considering this fact, the commonly accepted 25 ppm NO3-N critical level for manure- or legume-N fertilized soils may be too low for soils that have only received fertilizer N.

Leaching of soil nitrate is expected with ponding, flooding, or soil saturation, but not all of the nitrate will have been moved below the root zone. A shortcoming of the 1-foot sampling depth is that it does not always reflect plant available N deeper in the profile, particularly when abnormal leaching occurs. This is why we suggest also sampling from the 1- to 2-foot depth for assessment of soil N availability, particularly in sandy soils.

In our on-going N rate trials conducted throughout the state, the “normal” background levels of soil N in the upper 1 foot of mineral soils typically range from 5 to 10 ppm NO3-N and 4 to 8 ppm NH4-N for corn grown in rotation with soybean or corn without manure- or legume-derived N. Typically the deeper 1- to 2-foot soil samples would have slightly lower N levels.

Making a decision

We suggest that the 25 ppm NO3-N critical level for manure- or legume-N fertilized soils may be too low for soils that have only received fertilizer N and where N loss conditions have been severe. Where enough rainfall has occurred to cause substantial N loss, we suggest this level of rain has depleted the lower soil profile as well as the upper foot of soil.

Table 1 lists estimates of expected soil NO3-N levels with different fertilizer rates assuming “normal” background levels of nitrate and ammonium at the time of fertilization and a “normal” amount of movement below the one foot sampling depth (approximately 1/3 of the fertilizer N is moved below the 1-foot sampling depth but retained within the root zone with normal rainfall). If the corn is healthy and the growing season typical from here on out, we would suggest applying no more than 10 pounds of N for every 2 ppm reduction in soil sample N below the expected levels listed in the table.

Recognize that as a healthy crop moves through the rapid growth phase prior to pollination, soil N levels will naturally decrease in response to rapid N uptake by the plants. However, by the time a healthy crop reaches the V9 leaf stage (about 30 inches tall), only 19 lbs/ac N (equivalent to 5 ppm soil NO3-N in a 1-foot deep sample) have typically been taken up the plants (Mengel, 1995). However, by the time a healthy crop reaches shoulder-high (~ V15 or 60 inches tall), approximately 116 lbs/ac N (equivalent to 29 ppm soil NO3-N in a 1-foot deep sample) have been taken up by the plants.

The following examples give you an idea of how the tabular information may be used to make this decision.

Example calculation when only NO3-N is determined: Fertilizer N was applied at 160 pounds of N per acre in early April as 28% UAN in southern Indiana. Only soil NO3-N analysis was requested because it was assumed that most of the urea- and ammonium-N had been converted to nitrate since temperatures were warm prior to the April and May rains. The expected NO3-N level from the table below for a 160-lb N application is 35 ppm. Laboratory results indicated only 20 ppm NO3-N. The suggested N application rate would be: ((35 ppm – 20 ppm) / 2) x 10 = (15 ppm / 2) x 10 = 7.5 x 10 = 75 pounds per acre.

Example calculation when both NO3-N and NH4-N are determined: Anhydrous ammonia with nitrapyrin was applied at 160 pounds of N per acre in late April in northern Indiana. Since the N application was relatively recent and a nitrification inhibitor was used , both NO3-N and NH4-N analysis of soil samples were requested. The expected NO3-N plus NH4-N levels listed in the table for a 160-lb N application is 41 ppm. Laboratory results indicated 15 ppm NO3-N and 20 ppm NH4-N for a total measured N level of 35 ppm. The suggested N application rate would be: ((41 ppm – 35 ppm) / 2) x 10 = (6 ppm / 2) x 10 = 3 x 10 = 30 pounds per acre.

Sidedress N application rates

If no fertilizer N has been applied this season or soil N measurements suggest little N remains from fall- and spring-applied N, consider using our current N rate recommendations based on results of field trials conducted since 2006 throughout the state using efficient methods and timings of N fertilizer application. The estimated Agronomic Optimum N Rate (AONR) for fine textured soils in westcentral and northwest Indiana is about 173 lbs N / ac. The AONR for fine textured soils in northeast, eastcentral, and central Indiana is about 221 lbs N / ac. The AONR for the remainder of the state (NC, SW, SC, and SE) is about 183 lbs N / ac. For more details on these recommendations see our current publicationat: http://www.agry.purdue.edu/ext/corn/news/timeless/NitrogenMgmt.pdf

At the five Purdue locations where we conducted paired trials of corn following soy and corn following corn in 2007-2010, the average AONR for corn/corn was 44 lbs greater than for corn/soy. Based on $0.50 to $0.70/lb N and $7.00/bu corn, the average Economic Optimum N Rate (EONR) for corn following soybean was approximately 164, 203, and 172 lbs N / ac for WC+NW, NE+EC+C, and the remainder of the state, respectively. The EONR values for other combinations of N cost and grain price are listed in the Nitrogen Management Guidelines for Indiana or in the on-line N calculator for Indiana at this web site: http://extension.agron.iastate.edu/soilfertility/nrate.aspx.